1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a back light device for use in a LCD device.
2. Description of Related Art
Liquid crystal display (LCD) devices are in wide use as display devices capable of having reduced weight, size and thickness. Of these, active matrix LCD devices, in which thin film transistors (TFTs) and pixel electrodes are arranged in the form of a matrix, have been widely used due to high resolution and excellent performance in implementing moving images.
FIG. 1 is a cross-sectional view illustrating a liquid crystal panel of a typical active matrix LCD device. As shown in FIG. 1, the liquid crystal panel 20 includes lower and upper substrates 2 and 4 with a liquid crystal layer 10 interposed therebetween. The lower substrate 2 is divided into two regions: a region S; and a region P. TFTs are arranged on the region S as a switching element, and pixel electrodes 14 are arranged on the pixel region P. The upper substrate 4 includes a color filter 8 and a common electrode 12. Through the pixel electrode 14 and the common electrode 12, voltages are applied to the liquid crystal layer 10. In order to prevent a leakage of the liquid crystal, edge portions of the two substrate 2 and 4 are sealed by a sealant 6. The TFT receives signals from an external drive integrated circuit (IC) to drive the pixel electrode 14.
Such LCD devices are divided into transmissive LCD devices and reflective LCD devices. The transmissive LCD device includes a back light device for providing the liquid crystal panel 20 with light.
FIG. 2 is a schematic view illustrating a conventional transmissive LCD device having a back light device. As shown in FIG. 2, light generated from the back light device 30 directs toward the liquid crystal panel 20.
However, part of light generated from the back light device 30 is reflected from an edge portion “A” of a bottom surface of the liquid crystal panel 20, and reflected light causes a constructive interference along with light generated from the back light device. As a result, a portion “B” of the back light device 30 is relatively stronger in brightness than other portions around the portion “B”, whereby a bright line occurs along a dot line on an active area, i.e., a screen. In general, a bright line may occur even when a gap between a lamp housing and a light guide plate does not exist. This is the result of light that directs inward from the light guide plate does not conduct a total reflection and does not form a uniform light distribution before emitting from the light guide plate.
In order to prevent a bright line, several conventional solutions have been introduced. FIG. 3 shows one of conventional solutions to prevent a bright line. As shown in FIG. 3, a black pad 40 is formed along an edge portion of the top surface of the back light device 30. However, such a solution is very difficult to work. In addition, when an assembly error occurs between the liquid crystal panel 20 and either of the back light device 30 and the black pad 40, inferiorities result such as a bright line, a light leakage, and a covering of an edge portion of an active area. For example, when the black pad 40a is located outside a proper location, a bright line occurs along a dot line. Further, when the black pad 40b is located inside a proper location, the black pad 40b covers an active area.
FIG. 4 shows another solution to prevent a bright line according to the conventional art. As shown in FIG. 4, the back light device includes a lamp 31 as a light source. A lamp housing 39 surrounds most of the lamp 31. A lamp reflector 32 is located inside of the lamp housing 39. A light guide plate 33 is located adjacent to the lamp 31. Light emitted from the lamp 31 directs toward the liquid crystal panel (not shown) through the light guide plate 33. The light guide plate 33 has a plurality of patterns 33a (for example, a dot pattern) on its bottom surface. The patterns 33a are formed by a printing, a V-cut, molding or the like. A plurality of sheets are stacked on the light guide plate 33: a diffusing sheet 35; two prism sheets 36 and 37; and a protecting sheet 38. A reflector 34 is located under the light guide plate 33. On the reflector 34, a printing portion 34a is formed to prevent a bright line that comes from light incident to an upper or a lower surface of the light guide plate 33 other than an emitting portion of the light guide plate 34. The printing portion 34a is colored by mixing colors and a certain pattern of a gray scale.
However, the printing portion 34a of the reflector 34 may lower the brightness. Further, a brightness deviation may occur due to a variation of a printing portion size and an ink color, an assembly error, and a fluctuation of the reflector.
For the foregoing reasons, there is a need for a back light device that prevents a bright line and a liquid crystal display device having the same.